The present invention relates to a recording sheet feeding device for an image forming apparatus such as a copier, facsimile and printer, and more particularly relates to improvements in document conveyance in an automatic document feeder and recording sheet conveyance accommodated in a cassette or intermediate tray.
Conventionally, in an image forming apparatus such as a copier, facsimile and printer, a paper feed unit is provided that is operated in the following manner: the uppermost or lowermost sheet of a stack on a tray, cassette, or stacker is taken out, and the following sheets are taken out one by one.
For example, in order to timely transfer a toner image from a photoreceptor onto a transfer sheet, the transfer sheet is conveyed as follows: the transfer sheet is conveyed out from a paper feed cassette through a pickup roller and feed roller; the transfer sheet collides with a registration roller and is stopped at the registration roller under the condition that slack is provided to the sheet; the registration roller is driven synchronously with the start of a scanning operation of an optical system that reads document information; and then the transfer sheet is held by the registration roller so that it can be conveyed to the transfer region. In the case of a paper conveyance device disclosed in Japanese Patent Application Open to Public Inspection No. 73374/1975, the paper conveyance speed is controlled in the following manner: a vacuum feeder is provided in the device; a thickness detection lever and pressure detection lever that are displaced in accordance with the thickness and pressure of papers are provided to detect the thickness and pressure; and a motor to convey the papers is driven according to the results of the detection.
In a recording paper refeed device provided in an intermediate tray in the case of two-side recording or multirecording, recording papers are supplied and conveyed in the same manner as described above.
In an automatic document feeder, document sheets (referred to as a document, hereinafter) are conveyed out from a stack tray by a sheet feed device one by one; the document is conveyed onto a platen by a conveyance belt; the document is exposed by an exposure lamp; and then the document is conveyed onto a document discharge tray. According to a bottom sheet separation and conveyance device disclosed in Japanese Patent Application Open to Public Inspection No. 36248/1985, compressed air generated by an air injection means is supplied between the sheets in order to separate them. In this device, the air flow amount is controlled according to the amount of stacked sheets.
In a high speed copier that can copy at a high speed of not less than 50 sheets per minute, it is necessary to provide an automatic document feeder that can quickly and positively convey a document onto a platen glass, and further it is necessary to provide a recording paper feed device that can send a transfer sheet from a paper cassette or an intermediate tray at high speed. In the aforementioned automatic document feeder and recording sheet feed device, a large number of documents and recording sheets stacked on a tray must be positively conveyed at high speed without failure. Conveyance conditions are different according to the size and amount of documents or recording sheets to be conveyed. Therefore, conveyance tends to become unstable.
In order to prevent the aforementioned problems, a paper feed device provided with a vacuum belt or a vacuum feeder has been proposed. However, the mechanism of the aforementioned device is complicated and large-scale, and further noises are caused by the device. In the case of the aforementioned paper conveyance device disclosed in Japanese Patent Application Open to Public Inspection No. 73374/1975, the sheet pressing force can be changed only in 2 stages, therefore the device is not suitable for the complicated conveyance operation described above.